Spontaneous combustion may occur in piles of moist organic material where heat is generated in the early stages by the respiration of bacteria, molds, and microorganisms. A high moisture content is required for vigorous activity, and heating is generally controlled by maintaining the moisture content below a predetermined level. This type of heating can only raise the material to the temperature range of 50 to 75 degrees C (122 to 167 degrees F), where the living organisms die. Beyond this point, oxidation reactions must take over if ignition is to occur. The existence of biological heating requires careful control of moisture, air supply, and nearby combustible or flammable materials. If a "hot spot" in a pile of organic material comes in contact with a highly flammable liquid or gas, a fire or explosion may occur. Heat generated by biological action may also act as a catalyst for other reactions which occur only at elevated temperatures.
The likelihood of biological heating may be reduced by the following measures:
Coal presents hazards between the time it is mined and its eventual consumption in boilers and furnaces. Below are listed some of the characteristics of spontaneous fires in coal. These characteristics can be used to evaluate the potential for coal fires and as guidelines for minimizing the probability of a fire.
Coal should be stored in properly designed bunkers, silos, bins, or in outside piles. The most important aspects of coal storage are minimizing the flow of air through the pile, using the "first-in, first out" rule of thumb, and minimizing the amount of finely divided coal in the pile. "Hot spots" should be removed or exposed to the atmosphere to allow cooling. Coal should be compacted if possible to reduce the amount of air in the pile. Water may be used to cool hot spots, but should be used with caution on large areas of hot coal to present accumulations of hazardous amounts of water. Coal should not be stored in outside piles located over utility lines (water, gas, etc.).
There are several kinds of pyrophoric gases that should be included in any discussion of pyrophoricity. Many of these are used in manufacturing microelectronics. All of the gases presented here have 3 things in common: a) they can ignite immediately upon exposure to air, b) they are all nonmetallic hydrides, and c) many other compounds which contain these gases in their molecular structure are also pyrophoric.
Arsine (AsH{sub 3}), also know as arsenic hydride, is a colorless, highly toxic gas with a distinctive garlic-like odor. It is heavier than air and is a blood and nerve poison. Arsine will generally not ignite in air unless at elevated temperatures, but it can be detonated by a suitably powerful initiation (heat source, shock wave, electrostatic discharge). Arsine may also exist in other compounds. The ignition temperature of many of these arsine containing compounds is lower than that of arsine, causing them to ignite in air even at low temperatures (below 0 degrees C, 32 degrees F). All arsine compounds should be considered pyrophoric until they are properly characterized.
Diborane (B{sub 2}H{sub 6}) is a highly toxic, colorless gas with a repulsive but sweet odor; it is highly reactive and flammable. It forms flammable mixtures with air over a wide range (flammable limits, 0.9% and 98%). The ignition temperature of diborane is between 38 and 52 degrees C (100 and 125 degrees F). Diborane will ignite spontaneously in moist air at room temperature. It reacts spontaneously with chlorine and forms hydrides with aluminum and lithium, which may ignite spontaneously in air. It reacts with many oxidized surfaces as a strong reducing agent, and reacts violently with vaporizing liquid-type extinguishing agents.
Storage should be in a detached, refrigerated (less than 20 degrees C, 68 degrees F), and well-ventilated place. Boranes should be separated from halogens and other oxidizing agents and checked periodically for decomposition. Protect against electrical spark, open flames, or any other heat source. A dry nitrogen purge should be used in any transfer. Waste material should be completely hydrolyzed with water before disposal. Combustible solutions should be burned as a means of disposal. There are no special shipping requirements for diborane other than steel pressure cylinders.
Fire fighting should be done from an explosion-resistant location. Use water from unmanned monitors or hoseholders to keep fire-exposed containers cool. If it is necessary to stop the flow of gas, use water spray to protect personnel effecting shut-off. Halon should not be used as an extinguishing agent on diborane fires.
Phosphine (PH{sub 3}) is a highly toxic colorless gas. This chemical is very dangerous, with an ignition temperature of 212 degrees F, often igniting spontaneously. Phosphine gas readily combines with nitrates, halogens, and metals to form very explosive and volatile compounds. Specifically, phosphine reacts violently with air, BCl{sub 3}, Br{sub 2}, Cl{sub 2}, ClO, Hg(NO{sub 3}){sub 2}, NO, N{sub 2}O, NCl{sub 3}, -NO{sub 3}, N{sub 2}O, HNO{sub 2}, O{sub 2}, (K+NH{sub 3}), and AgNO{sub 3}. In addition, at elevated temperatures, phosphine decomposes, emitting highly toxic fumes of PO{sub x}, which react vigorously with oxidizing materials. It possesses the characteristic putrefying odor of a mixture of garlic and decaying fish. Prolonged exposure to very low concentrations will cause chronic poisoning, characterized by anemia, bronchitis, gastro-intestinal disturbances, and visual, speech, and motor difficulties.
Silane (SiH{sub 4}), also know as silicon tetrahydride, is a colorless gas with a putrid odor. It and its compounds (e.g., disilane Si{sub 2}H{sub 8}) can ignite in air and react violently with chlorine (Cl{sub 2}). The presence of other hydrides as impurities causes ignition always to occur in air. However, 99.95% pure silane ignites in air unless emerging at very high gas velocities, whereas mixtures of up to 10% silane may not ignite. Hydrogen liberated from its reaction with air (atmospheric oxygen) often ignites explosively. Silanes react violently with chlorine and bromine. All silanes should be considered pyrophoric until they are properly characterized. Halon should not be used as an extinguishing agent on silane fires.
Pyrophoric gases may spontaneously explode at high gas release rates. For fires involving flammable gases, the best procedure is to stop the flow of the gas before attempting extinguishment of the fire. To extinguish the fire while allowing continued flow of the gas is extremely dangerous; an explosive cloud of gas/air mixture may be created that, if ignited, may cause far more damage than the original fire. Extinguishing the flame using carbon dioxide or dry chemical may be desirable to allow immediate access to valves to shut off the flow of gas, but this must be done carefully. In many cases, it will be preferable to allow continued burning, while protecting exposures with water spray, until the flow of gas can be stopped. Since many pyrophoric gases react violently with halogens, Halons should not be used as extinguishing agents.
Pyrophoric gas cylinders in storage or dispensing areas should be provided with the following safeguards:
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Distance to Distance to
Fence in Ft Wall in Ft
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Unconfined Cylinders 6 9
Single Cylinder Cabinets 12 12
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NFPA 70, National Electrical Code, should be consulted for explosion
proofing
of electrical equipment near flammable gas storage areas.
Hydrazine is a colorless oily liquid resembling water in appearance and possesses a weak, ammonia-like odor. Its chemical formula is N{sub 2}H{sub 4}. Commercially it is available as an anhydrous (without water) liquid and in aqueous solutions. Hydrazine is most well known for its use as a rocket fuel, but is also used in manufacturing agricultural chemicals, explosives, and plastics. It fumes in air and reacts with all oxidizing agents. Hydrazine is hypergolic, meaning that it reacts explosively upon contact with many oxidizing agents. The flash point of hydrazine is 38 degrees C (100 degrees F). Its autoignition temperature is 270 degrees C (518 degrees F) on a glass surface but may be as low as 23 degrees C (74 degrees F) when in contact with a strong oxidizing agent. Hydrazine forms flammable mixtures with air from 4% to 100% by volume and decomposes when heated. Hydrazine ignites in air at room temperature when exposed to metal oxide surfaces and in a wide variety of porous materials.
Storage in a detached building is preferred. Inside storage should be in a standard flammable liquids storage warehouse, room, or cabinet. An emergency water reservoir or sprinklers should be provided for fire extinguishment. Hydrazine should be stored separately from metal oxides, acids, and all oxidizing agents. Hydrazine is highly toxic and may be fatal if inhaled or absorbed through the skin. It is also corrosive and may cause severe eye and skin burns. Protective clothing that prevents penetration of hydrazine and positive pressure self-contained breathing apparatus must be worn when working with hydrazine.
Fires involving hydrazine may produce irritants and toxic gases such as nitrogen oxides. Fires should not be approached without protective clothing and positive pressure respirators. Hydrazine fires should be approached from upwind to avoid hazardous vapors and toxic decomposition products. Flooding amounts of water should be applied as a fog or spray. Water should be sprayed on fire-exposed containers of hydrazine to keep them cool. Fires should be fought from a protected location or at a maximum possible distance. Flooding amounts of water may be necessary to prevent reignition.
There are two different compounds of phosphorus (P{sub 4}). These are commonly known as white (or yellow) phosphorus, and red phosphorous. Red phosphorus is not considered pyrophoric. However, red phosphorus ignites easily and produces phosphine (a pyrophoric gas) during combustion.
Pyrophoric (white, or yellow) phosphorus is a colorless to yellow, translucent, nonmetallic solid. It ignites spontaneously on contact with air at or above 30 degrees C (86 degrees F). Phosphorous is explosive when mixed with oxidizing agents. Fumes from burning phosphorus are highly irritating but only slightly toxic except in very high concentrations. Like red phosphorus, white phosphorus also produces phosphine during combustion.
When storing, protect containers against physical damage. Phosphorus should always be kept underwater, or under an inert atmosphere, separated from oxidizing agents and combustible materials. When shipping, keep phosphorus under water in hermetically sealed cans inside wooden boxes, under water in drums, or in tank motor vehicles or tank cars under water or blanketed with an inert gas.
Phosphorous fires should be deluged with water until the fire is extinguished and the phosphorus has solidified. The solidified phosphorus should then be covered with wet sand, clay, or ground limestone.